Think, 6 players.
Initial startup consists of picking a primary and secondary weapon and that's it. You're tasked to survive rounds against the Ceph and Cell enemies. Each kill rewards you with an amount of points (depending upon how you killed them be it with a headshot, thrown object etc.) At the end of each round of enemies you can spend those points on picking up an attachment for your weapons, buying grenades, refilling ammo or saving them up to spend on a suit module. These modules would just be the copy-paste ones from the MP so no need to worry about that.
From there, you can continue to fight through these rounds and occasionally you'll get an objective. You'd need to defend a sequence of drop pods that land, defend a single spire, retrieve a power cell or two, kill a particular enemy. The list could go on and on.
You could continue to kit your guy out with different weapons and attachments or you could continue to unlock the additional tier of the suit module as you progress.
Before the match starts, a server could select a particular difficulty and number of rounds until the game is complete.
In truth? I don't mind if this is simply some £5 DLC added on. I would love to play this mode with my friends and it would add some longevity to the entire experience as a whole.
Thoughts, suggestions, whatever is welcome (as long as it's constructive). I'm sure this is very much a pipe dream but hey, worth a go anyway.
Kind regards.